1. Field of the Invention
The present invention relates to a writing utensil such as a ball pen and, more particularly, to a writing utensil having an ink holder unit that can be readily inserted into and extracted from a cooperating housing/main shaft. The invention is also directed to a method of manufacturing such a writing utensil.
2. Background Art
A ball pen is known having an ink holder unit operatively held in a cylindrical housing/main shaft with a writing tip on the ink holder unit projecting from the main shaft and being held by a tip stopper secured to the main shaft. In such a conventional writing utensil, an external screw thread is formed on the main shaft to cooperate with an internal screw thread formed on the tip stopper. The main shaft is joined to the tip stopper by coupling the external screw thread of the main shaft and the internal screw thread of the tip stopper.
With the above structure, one can easily extract the ink holder unit, and thus, only the ink holder unit need be drawn out of the main shaft to replace it with a new one whenever the stored ink is exhausted. Since the main shaft can be repeatedly re-used, it is quite economical and preferred for business use. An invention on a ball pen of this kind is described in Japanese Patent Laid-Open Publication No. 51996/1989.
Nevertheless, coupling the external and internal threads of the ball pen having the above structure is quite troublesome, and thus, the above structure is not well suited for mass production.
The reason for coupling the external and internal screw threads is to prevent the ink holder unit from slipping out of the main shaft. There is also known a ball pen which prevents the ink holder unit from slipping out of the main shaft by way of fixing the main shaft and the tip stopper with an adhesive agent instead of coupling threads. Since the main shaft and the tip stopper are merely coated with an adhesive agent prior to a simple press fit step, this structure is well suited for mass production. An invention on a ball pen of this kind is described in Japanese Patent Laid-Open Publication No. 179298/1994.
Nevertheless, since this method of fixing the main shaft and the tip stopper with adhesive agent will not allow extraction of the ink holder unit, this method cannot be used with a ball pen to allow the ink holder unit to be replaced.
Thus, there has been a demand for such a writing utensil i.e. ball pen, having structure to allow replacement of only the ink holder unit without necessarily executing a screwing step as part of the assembly process.